1. Field
One embodiment of the invention relates to a phase change recording medium which records information by reversibly changing the state by irradiation with a light beam. The present invention more particularly relates to a phase change recording medium in which the atomic arrangement of a thin film for holding recorded information changes between an amorphous state and crystalline state.
2. Description of the Related Art
(Phase Change Optical Recording Principle)
In a phase change optical recording film, a portion heated to the melting point or more generally melts and takes an amorphous atomic arrangement when rapidly cooled. The recent researches indicate the possibility that this amorphous atomic arrangement is not a complete amorphous state but has a short range regularity. However, XRD (X Ray Diffraction) measurements show that there is no peak which is observed if a crystal exists. Therefore, a long range regularity which is the essential characteristic of a crystalline state does not exist or is very weak. Accordingly, “a state in which a portion heated to the melting point or more takes an amorphous atomic arrangement when rapidly cooled” will be referred to as an amorphous state hereinafter as usual. Also, when a material is held for a predetermined time or more in a temperature region from the crystallization temperature to the melting point, the material remains crystalline if it is initially crystalline, but crystallizes if it is initially amorphous (a solid phase erase mode). Depending on the material of a recording film, it is also possible to crystallize an amorphous portion of a recording film by melting the portion by heating it to the melting point or more, and then gradually cooling it (a melt erase mode).
Since the intensity of reflected light from an amorphous portion differs from that of reflected light from a crystal portion, the intensity of the reflected light is converted into the intensity of an electrical signal, and information is read out by A/D converting the electrical signal. This is the principle of the phase change recording medium. It is also possible to record and read out information by using the transition between a metastable crystalline phase such as a martensite phase and a stable crystalline phase, or between plurality of metastable crystalline phases, instead of the crystal amorphous phase change.
(Method of Increasing Density)
The amount of information to be recorded on one recording medium, i.e., the recording capacity can be increased by the following two methods. One is a method which decreases the pitch of recording marks in the track direction. If downsizing advances to a certain degree, however, recording marks become smaller than the size of a light beam for playback, so a playback beam spot may temporarily contain two recording marks. If the recording marks are well separated from each other, the playback signal is largely modulated, so a large amplitude signal is obtained. If the recoding marks are close to each other, however, a small amplitude signal is obtained, so an error readily occurs upon conversion into digital data.
The other recording density increasing method is to decrease the track pitch. This method can increase the recording density without being largely influenced by the reduction in signal intensity caused by downsizing of the mark pitch. However, this method has the problem that in a region where the track pitch is equal to or smaller than the size of a light beam, information in a certain track deteriorates while information is written in or erased from an adjacent track. That is, so called cross erase occurs.
The causes of the cross erase are that the edge of a laser beam on an adjacent track directly irradiates a mark, and that a heat flow during recording flows into an adjacent track and raises the temperature of a mark on the adjacent track, thereby deteriorating the shape of the mark. It is necessary to solve these problems in order to increase the density of the phase change recording medium. Also, to accurately read a downsized mark and decrease the probability of a read error at the same time, it is desirable to smoothen the edge of a recording mark to be formed and minimize the noise component.
(Increasing Capacity by Multilayered Medium)
Another method of increasing the capacity is to form a plurality of information recording layers and stack them. Jpn. Pat. Appln. KOKAI Publication No. 2000-322770 discloses this method. A medium designed to read/write information from/on one surface by stacking two layers is called a single side dual-layer medium, or simply called a dual-layer medium. In this single side dual-layer medium, an information layer (to be referred to as L0 hereinafter) formed close to the light incident side must have a transmittance of about 50% or more so as not to decay light more than necessary in L0 when accessing a far information layer (to be referred to as L1 hereinafter). For this purpose, the thickness of a recording film in L0 must be very thin, i.e., 10 nm or less.
Since the film is thin, the holding time necessary for crystallization prolongs, so incomplete erase occurs at a normal rewriting (or overwriting) rate. The proceedings of the 12th Symposium on Phase Change Optical information Storage (Proceedings of PCOS2000), pp. 36-41 disclose that a method of substituting a portion of a GeSbTe recording film with Sn is effective to solve the above problem. Likewise, Jpn. Pat. Appln. KOKAI Publication No. 2001-232941 discloses that it is effective to partially substitute a GeSbTe recording film with Bi, In, Sn, and Pb. To ensure the erase ratio described above, however, it is unsatisfactory to improve the recording film material alone, and necessary to form a film having a crystallization promoting effect in the interface with the recording film.
According to Proceedings of PCOS2000, pp. 36-41, germanium nitride (GeN) is effective as “an interface film having a crystallization promoting effect”. However, the present inventors made extensive studies and have found that cross erase occurs and the track pitch cannot be well decreased if the conventional interface film material such as GeN is combined with a very thin film having a film thickness of 10 nm or less of the recording film material described above. The present inventors also made extensive studies and have found that silicon carbide (SiC) reportedly having a crystallization promoting function increases the light attenuation coefficient at a wavelength of 405 nm of a laser beam used for a next generation, high density optical disk, and produces a very large optical loss. In addition, germanium nitride (GeN) and silicon nitride (SiNx) also produce optical losses. On the other hand, a medium having no interface film can suppress recrystallization of the molten portion and reduce cross erase, but has a totally insufficient erase ratio.
(Method of High Speed Recording)
High speed recording is another requirement for phase change optical recording. When recording an image or move, for example, if the image or move can be recorded within a time shorter than the actual playback time, it is possible to facilitate dubbing of a distributed medium or implementation of a so called time shift function which allows the user to watch previous images or moves by tracing back the time during recording of broadcasting. One cause which interferes with high speed recording in phase change recording is the problem that information is incompletely erased when a laser having a relatively low erase level performs crystallization during overwrite, i.e., the problem of an insufficient erase ratio. That is, a recording mark passes through a laser spot at a high speed and hence does not stay in a temperature region capable of crystallization for a sufficiently long time, so information is incompletely erased.
Jpn. Pat. Appln. KOKAI Publication No. 11-213446 discloses a method of increasing the erase rate by promoting crystallization by forming materials such as GeN in the interface with a recording film. However, the present inventors conducted experiments by using the materials disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-213446 as an interface film without controlling the state of a recording film or the interface state, and have found that the molten portion partially recrystallizes during recording, i.e., to form a recording mark having a necessary size requires melting a range larger than the necessary size. The use of this interface film melts a region more than necessary and therefore accelerates cross erase described earlier, i.e., has a reverse effect from the viewpoint of high density recording. In other words, if information is recorded with a laser power within the range allowable from the point of view of cross erase, the width of the formed recording mark decreases, and the obtained carrier to noise ratio (CNR) lowers. On the other hand, a medium having no interface film can suppress recrystallization of the molten portion and reduce cross erase, but has an entirely insufficient erase ratio. Accordingly, a demand has arisen for a new interface film material capable of suppressing recrystallization of the molten portion during recording while increasing the crystallization rate during erase.
(Film Design of Phase Change Recording Medium)
In the phase change recording medium, as explained in “Phase Change Optical Recording Principle”, an amorphous mark is formed (i.e., data is written) in a desired portion of a recording film by irradiation with a laser pulse, or data is erased by crystallizing an amorphous mark by irradiating it with a low power laser. In the former process, an amorphous mark is formed by rapidly cooling a portion irradiated with the laser. In the latter process, an amorphous portion is crystallized as it is gradually cooled. Also, the larger the laser absorbance of the recording film, the lower the laser power necessary to record or erase data; the smaller the absorbance, the larger the laser power necessary to record or erase data. The absorbance of the recording film is determined by the optical characteristics and thermal characteristics of each film material of the medium formed by a multilayered film. For example, it is possible to change the arrangement by the selection of film materials equal in absorbance, and produce the anisotropy of thermo-physical properties between a rapid cooling structure and slow cooling structure or between the longitudinal direction and sectional direction of a film.
That is, film design of the phase change recording medium includes optical design and thermal design. For optical design, it is necessary to grasp the optical characteristics of each thin film. For thermal design, it is necessary to grasp the thermo-physical properties such as the melting point, melting latent heat, and crystallization temperature of each thin film. The optical constant of a thin film can be measured by using an apparatus such as an ellipsometer. Several researches have implicitly indicated that the thermo-physical properties of a nanometer order film are different from bulk thermo-physical properties. However, it is impossible to systematically measure them (the thermo-physical properties of a thin film and bulk thermo-physical properties) while removing the effects of other factors. Therefore, empirical parameters are necessary to correct them (correct the effects of the other factors in order to grasp the thermo-physical properties of a thin film). In particular, there is almost no method of measuring the interface thermal resistance or boundary thermal resistance between nanometer order films. The present inventors made extensive studies on these problems as well, and have established a thermal designing method which takes account of the thermo-physical property values of a thin film and the boundary thermal resistance between thin films measured by a highly accurate method using thermal design, thereby completing this invention.
(Interface Layer Materials)
It is disclosed by, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-6794 “a technique which mixes a carbide or nitride in several oxides such as Ta2O5” which aims a sulfur (S) free protective film material, as a known technique which can be an interface layer material having the crystallization promoting function, instead of GeN. The main purpose of Jpn. Pat. Appln. KOKAI Publication No. 2003-6794 is to improve a current DVD using a laser diode having wavelength λ=650 nm. The material of Jpn. Pat. Appln. KOKAI Publication No. 2003-6794 becomes opaque and increases the optical loss when the next generation blue-violet laser diode (λ=405 nm) is used. Therefore, this material has the problem in the next generation, high density medium. GeN described above also becomes opaque and increases the optical loss at λ=405 nm.
Also, Jpn. Pat. Appln. KOKAI Publication No. 2003-323743 discloses a technique concerning (ZrO2)M(Cr2O3)100-M, i.e., a Zr Cr O system, as a known technique of an interface layer material containing ZrO2. M shows concentration of ZrO2 in the comcound. Although Cr2O3 is mixed in this material system, the material has a very large attenuation coefficient in the visible light wavelength region, especially λ=405 nm. Therefore, a thin film having a relatively large attenuation coefficient is formed if the material is a mixed material contained, albeit in a small amount, in the film.
(Material Systems of Recording Film)
A eutectic system recording film uses the melt erase mode in the erase process as described previously, so a cap layer is not required to have the crystallization promoting function. Therefore, details of the recording film such as the film material and micro-structure have not been examined. In addition, since the eutectic system uses the melt erase mode as described above, it is very difficult to perform so called land and groove recording which performs information recording and playback for both a land (L) and groove (G). This is very disadvantageous to increase the recording density.
By contrast, a so called pseudobinary system recording film material such as Ge2Sb2Te5 can rapidly change its phase from an amorphous state to a crystalline state in a solid phase state without taking the melt erase mode (the solid phase erase mode). If the recording film is thin, however, the time required for crystallization relatively prolongs, so it is essential to increase the crystallization rate by controlling the state of the recording film, or use an interface layer material having the crystallization promoting function. This achieves land and groove recording.
As described above, very large amounts of phenomenological findings, crystallographic findings, and findings of the bulk thermo-physical properties and chemical properties concerning the recording film using the phase change system and the interface layer material have been accumulated, and applied to the research, development, and design of media. Presently, however, there is almost no microscopic research or almost no research on, e.g., the electron state of a material.
S.K. Bahl et al. tried to examine the electron state of GeTe which presently has various problems as a phase change recording film material and hence is presumably hardly used (J. Appl. Phys., Vol. (1970), p. 2,196). The research by S. K. Bahl et al. is based on a simple band model from an electron transport phenomenon such as the temperature dependence of the electrical resistivity, and aims at estimating a rough change in band structure between the crystalline state and amorphous state. However, since the research is based upon the simple band model and experimental data is the electron transport phenomenon alone, S. K. Bahl et al. proposed only a very simple band model. This research of course does not contribute to an application to a phase change recording medium.
Also, Ogawa et al. tried to estimate, by calculations, the electron state of Ge2Sb2Te5 as a material usable in a phase change recording medium (Proceedings of PCOS1997, pp. 50-53). That is, Ogawa et al. tried to calculate the band structure from the crystal structure on the basis of, e.g., the temperature dependence of the electrical resistivity, but failed to compare the calculation results with the experimental facts, and hence could not apply the material to a phase change recording medium.
In Jpn. Pat. Appln. KOKAI Publication No. 2000-322770 or 2003-323743, Proceedings of PCOS2000, pp. 36-41, or Proceedings of PCOS1997, pp. 50-53, the phase change information recording medium which performs high speed, high density recording has the problem of recrystallization of the molten region during recording. This readily causes occurrence of cross erase which readily interferes with land and groove recording. Also, it is difficult to implement a high density, large capacity phase change recording medium capable of high speed overwrite which can assure a high crystalline/amorphous contrast and a high CNR even when a short wavelength laser (λ=405 nm or less) is used, has a sufficiently high erase ratio at a high linear velocity, and is superior in overwrite (OW) cycle characteristics and environmental resistance.